Prior art commutator type electric machines comprise a rotor, a stator and a commutator, integral with the rotor, on which a plurality of brushes are active defining sliding electrical contacts. These sliding electrical contacts enable a connection between the rotor windings, fitted to the rotor, and an electrical circuit which may be connected to the electricity mains supply that may also be connected to stator windings if the stator is not of the permanent magnets type.
The power supplied by the commutator electric machines is directly linked to the current circulating in the rotor windings, which, therefore, passes through the commutator and the brushes. The current passing through the brushes, which also has high nominal values, heats the brushes due to the Joule effect, and the brushes heat further due to their sliding on the commutator bars.
Electric motors of the conventional type comprise special brush holder elements, also known as brush holder casings, which are integral with a support element made of plastic material supported by the stator and by a casing outside the electric machine. Prior art brush holder elements have a tubular shape extending in a radial direction with respect to the rotor, that is to say, in a direction perpendicular to the axis of rotation of the rotor. The brushes slide inside the respective brush holder element under the action of a contrast spring which keeps an end of the brush pressed against the commutator to avoid reciprocal displacements between brush and commutator. Usually, each brush is electrically connected to the supply circuit by the same brush holder element, made of metal. The brushes are generally made of conductive material, in particular graphite possibly combined with other materials, such as phenolic resin or metal powders.
In addition, the support element is packed with the stator and held in position against the stator by a thrust force imparted by a cover outside the casing. The support element, which, as already mentioned, is made of plastic material, has elastic protrusions also made of plastic material, active between the support element and the stator to absorb thermal deformations or assembly tolerances of the support element.
Due to the known shape of the electric motors of the closed type, the brush holder element is immersed in a sealed environment subjected to continuous heating due essentially to the Joule effect caused by the passage of current through the rotor windings (and, possibly, the stator windings, too). This sealed environment is not subject to an air exchange, and, under steady-state conditions, the temperatures in this environment may be as high as 200° C. or more. This seriously penalises the heat dispersion of the brushes, which therefore tend to overheat, reaching unacceptable temperatures for their correct operation.
It has actually been found that the materials ordinarily used on a large scale to manufacture the brushes have wear resistance and electrical conductivity properties which remain at acceptable levels up to temperatures which are much lower than those reached in sealed electric motors and in particular these properties are maintained for temperatures up to approximately 150° C. When this limit has been exceeded, the decay of the electrical performance of the brushes as well as their deterioration (wear) increases in an unacceptable manner, and this leads to rapid wear of the electric machine which, after a short time, must be replaced or undergo maintenance.
Where the brush holder element is made of plastic material, the negative effect of the temperature increase is even more penalising due to the very poor heat exchange properties of the plastic materials. A brush holder element made of plastic material results in the brush operating almost in a situation of thermal insulation with respect to the outside, thus causing the brush to reach even higher temperatures compared with the case of metal brush holders.
For this reason, sealed motors are currently known in which the brushes and the brush holder elements are oversized compared with the dimensions they would have if installed in an open electric machine, that is to say, in a machine in which there is an exchange of air drawn from the outside, so as to reduce the overheating due to the Joule effect and to favour the heat exchange with the environment inside the electric machine. Disadvantageously, the above-mentioned over sizing has a negative influence both on the dimensional aspects of the electric machine, making it larger, and on the manufacturing cost of the machine itself.